I don't see that anyone frowned on not killling the dog. What was frowned on was the unjustified "warning" shot.
This ^^^^^
Jeff Cooper's Rules of Gun Safety:
RULE I: ALL GUNS ARE ALWAYS LOADED
RULE II: NEVER LET THE MUZZLE COVER ANYTHING YOU ARE NOT WILLING TO DESTROY
RULE III: KEEP YOUR FINGER OFF THE TRIGGER UNTIL YOUR SIGHTS ARE ON THE TARGET
RULE IV: BE SURE OF YOUR TARGET
Firing an intentional warning shot at any aggressive foe is inconsistent with Rule II as it shows a predetermined unwillingness to destroy a given hostile target.
I "frown" upon firing a handgun with a deliberate intent to scare. I also "frown" upon harboring a belief that firing warning shots at aggressive dogs on two occasions prepares one for the mental, mechanical, and tactical aspects of defending one's life against a violent two-legged threat. I submit that such a misguided mindset actually puts a person at a greater disadvantage in that it creates false senses of security and confidence in one's abilities. Thinking that you are ready to defend against an armed threat merely because you shot at some dogs is analogous to bringing a vehicle up to 100mph on the highway and subsequently believing you're ready to drive in a NASCAR race.
As to what I would have done ... I would have shot the dog dead, notified LEO, explained the situation, and detailed the threat I faced which justified my actions.
I had a similar situation happen this past week, where a large unleashed dog bounded up onto my back porch, knocked down my wife, cornered my daughter, and tried to attack my terrier. The dog was aggressive, and fortunately did not seriously injure my wife, child or terrier. I was not home at the time, but upon getting a hysterical call from the Mrs., I made it back post haste and explained to the neighbor about leash laws and the ramifications of a second incident. As my neighborhood is residential (and the DA lives two houses down), any discharge of a weapon is prohibited, thus dealing with an aggressive animal requires a different tact. I explained to the neighbor that if his dog (150lbs+) was to repeat his aggressive actions, I would defend my family and terrier with a 36" steel pry bar hanging from a post on the porch, having no compunctions about breaking his dog's neck or crushing its skull to quell an attack. Trying to scare this dog would be fruitless based on its size and aggressive nature, thus deadly force would be required. That said, defending my family by beating an aggressive dog with a pry bar in no way would prepare me to ward off violent home invaders, car jackers, etc., nor am I foolhardy enough to think that it would.